1. Field of the Invention
This invention relates to an antenna system for operation at frequencies in excess of 200 MHz, and particularly, but not exclusively, to an antenna system comprising an antenna having helical elements on or adjacent the surface of a dielectric core for receiving circularly polarised signals.
2. Discussion of the Related Art
U.S. Pat. No. 7,002,530 discloses a cylindrical dielectric-loaded antenna having a plurality of helical elements arranged on the outer cylindrical surface of a dielectric core. The helices are connected to each other at a distal end of the dielectric core by a link conductor which is arranged around the circumference of the distal end of the core. At a proximal end of the core, the helical antenna elements are connected to a pair of conductors, positioned on a circuit board mounted to the proximal end of the core. The circuit board comprises a phase splitting circuit which produces a single-ended output. The antenna elements being fed at the proximal end of the antenna, this antenna is an “end-fire” antenna.
In many mobile telecommunication applications, common-mode conducted noise interference can be a significant problem due to high-power interference sources. For example, in mobile telephone applications, planar inverted-F antennas (PIFAs) stimulate large currents in the ground plane. This problem is exacerbated by the fact that designers often want the board to radiate and the ground-plane is often placed on the top layer of a circuit board. Therefore, if a received signal is provided from a single-ended output, as an input to an amplifier, the amplifier will amplify common-mode noise signals present on the ground plane of the device in which the antenna is mounted. The amplified signal will therefore be distorted by common-mode noise.
United Kingdom Patent No. 2292638, in the name of the Applicant, discloses a further example of a dielectric-loaded helical antenna. The antenna has a plurality of helical antenna elements arranged on the cylindrical surface of the dielectric core. The helical antenna elements are fed at the distal end of the dielectric core by a feeder structure which is arranged along the axis of the dielectric core. As such, the antenna is a “backfire” antenna. The antenna also has a conductive sleeve formed on a proximal end portion of the dielectric core and which performs the function of a balun trap. The balun converts unbalanced signals at the proximal end of the antenna to balanced signals at the distal end of the antenna. The main advantages of this antenna are good isolation from the structures in which it is mounted and improved radiation patterns. The balun trap on the proximal end of the core of the dielectric isolates the antenna elements from the transmission line, preventing common-mode noise signals interfering with the amplification circuitry. The antenna is coupled to a short length of shielded transmission line such as a coaxial cable. Common-mode noise currents conducted along the outer sleeve of the coaxial cable are choked by the balun trap, preventing them entering the coaxial cable at its connection with the antenna proximal end.